1. Field of the Invention
This invention relates to an audio recording device which provides easy editing of voice and sound to be recorded on a floppy disk on which video signals are recorded.
2. Description of the Prior Art
There is generally known a device which records a still picture video signal on a magnetic floppy disk, such as an electronic still camera, and a device which uses a floppy disk of approximately 47 mm in diameter and approximately 40 .mu.m in thickness has been standardized and practically used. The floppy disk is rotated at a speed of 3,600 rpm to record 50 frames of still pictures on 50 tracks in a frame recording mode, or 25 frames of still pictures on 50 tracks in a field recording mode. The track width is 60 .mu.m and the track pitch is 100 .mu.m, and a guard band of 40 .mu.m in width is provided between tracks. Reproduced pictures can be displayed on a television screen, or printed out by a printer to obtain hard copies.
A time axis compression is used when recording an audio signal on such a floppy disk, and a time axis expansion is used when reproducing the audio signal from the floppy disk. Since the floppy disk makes one revolution in 1/60 second, only 1/60 second an audio can be recorded per track without time axis compression. By compressing the time axis to 1/640, for example, a sound signal of about 10 seconds can be recorded on a track. The sound signal includes a variety of signal components within the sound frequency bank such as a narrative description of the still picture recorded, background music for the reproduced picture, and the background sound present when photographing a scene with the electronic still camera.
FIG. 2 is a block diagram of a typical audio signal recording device, and FIG. 3 is a block diagram of an audio signal reproduction device.
Referring to FIG. 2, a sound signal 3 supplied from a microphone 1 to an input terminal 2 is passed through a low-pass filter (LPF) 4 and a noise reduction circuit (NR) 5, and then inputted to a time axis compression device 6. The time axis compression device 6 comprises an A/D converter 7, a RAM (random access memory) 8, a D/A converter 9, an address counter 10, a write clock generator 11, and a read clock generator 12. The address counter 10 is connected to a recording start switch 13 and a PG detecting device 14. For a time compression factor of M, the frequency of a read clock signal 12a is set to M-times a frequency fs of a write clock signal 11a, that is, M.fs. A sound signal 5a from the noise reduction circuit 5 is converted to a digital sound signal 7a by the A/D converter 7 with a sampling frequency of fs. The digital sound signal 7a is stored in the RAM 8 after the recording start switch 13 is turned on and with the address advanced by the address counter 10 having a write clock frequency of fs. Numeral 10a indicates an address signal. After the digital sound signal 7a is recorded in a predetermined location of the RAM 8, the address counter 10 advances the address with a read clock frequency of M.fs to read the contents of the RAM 8. A digital sound signal 8a read from the RAM 8 is converted to an analog sound signal 9a by the D/A converter 9 with a clock frequency of M.fs. The thus obtained analog sound signal 9a represents the sound signal 3 applied to the input terminal 2 with a time axis compressed to 1/M.
The time-compressed analog signal 9a has its high frequency range emphasized by a pre-emphasis circuit 15, modulated by a frequency modulator 16, and is then recorded on an appropriate track of a floppy disk 19 through a recording amplifier 17 and a magnetic head 18. Numeral 20 indicates a motor for rotating the floppy disk 19.
The time compression factor is determined by a frequency band which can be recorded on the floppy disk 19 and an upper limit frequency required for recording the sound signal. Electronic still cameras or the like can record by FM modulating a 6 MHz carrier with a sound signal of 3.2 MHz after compression on a floppy disk which rotates at a speed of 3,600 rpm. Where the upper limit frequency is fv (KHz), the compression ratio is given as M=3,200/fv. A sound signal which can be recorded is given as T=M/60=3,200/60fv. Table 1 shows the values for fv=2.5KHz, 5KHz, and 10KHz.
TABLE 1 ______________________________________ Read Upper Write clock limit Record- Compres- clock M. fs frequency ing time sion ratio fs (KHz) (MHz) Mode fv (KHz) T (sec) M (times) example example ______________________________________ 10 sec 5 abt. 10 640 10 6.4 5 sec 10 abt. 5 320 20 6.4 20 sec 2.5 abt. 20 1,280 5 6.4 ______________________________________
In the reproduction circuit, referring to FIG. 3, when the reproduction switch 32 is turned on, a signal 21a is inputted from a magnetic head 21 to an amplifier 22 where it is amplified, demodulated by a demodulator 23, inputted to a de-emphasis circuit 24 to de-emphasize its high frequency range, and inputted to a time axis expansion circuit 25. The time axis expansion circuit 25 comprises an A/D converter 26, a RAM 27, a D/A converter 28, an address counter 29, a write clock generator 30, and a read clock generator 31. The address counter 29 is connected to reproduction switch 32 and a PG detecting device 33. The frequency of a write clock signal 30a is the same as that of the read clock signal 12a in the time axis compression device 6 of the recording device shown in FIG. 2, that is, M.fs. The frequency of a read clock signal 31a is the same as that of the write clock signal 11a in the time axis compression device 6, that is, fs. A time-compressed analog signal 24a obtained in the de-emphasis circuit 24 is converted to a digital sound signal 26a by the A/D converter 26 with a sampling frequency of M.fs. The digital sound signal 26a is stored in the RAM 27 with the address advanced by the address counter 29 having a clock frequency of M.fs synchronizing with a PG signal 33a from the PG detecting device 33. Numeral 29a indicates an address signal. When the digital sound signal 26a is completely stored in the predetermined location of the RAM 7, the address counter 29 advances the address with a clock frequency of fs to read the contents of the RAM 27. A digital sound signal 27a read from the RAM 7 is converted to an analog sound signal 28a by the D/A converter 28 with a clock frequency of fs. The thus obtained analog signal 28a is a sound frequency band signal with the original time axis. The sound signal 28a is applied through a low-pass filter 34, a noise-reduction circuit 35, and an amplifier 36, to a speaker 37, and outputted as a sound.
In the above-described prior art technology, after a sound signal has been once recorded on the floppy disk 19, it has been difficult to change the recorded contents. To change the recorded contents, the sound signal recorded on the floppy disk 19 must be erased, and then a new sound signal must be recorded. The new sound signal must be reproduced from the floppy disk 19 to check if it is correctly recorded. Further, to record a combination of sound signals, for example, to record music for the first 5 seconds and a human voice narration for the next 5 seconds, the music must be played on the microphone 1 for 5 seconds, and the narrator must begin to speak immediately after the music. Thus, it is difficult to match the timing to switch from the music to the narration, and a mismatched timing or a blank period of no sound may result. If this is the case, the recording operation must be performed again, requiring a troublesome operation.
Further, in the prior art technology, to record background music and a narration in an overlapped fashion, the background music and narration must be recorded simultaneously, which has required a troublesome operation. For such a case, it would be convenient if there is a so-called "sound-on-sound" function which enables narration recording while hearing the background music and matching the timing.